Page 80 - MSAM-1-3

Materials Science in Additive Manufacturing Increasing density and strength in binder jetting

is selectively deposited with an array of nozzles that eject al. analyzed the impact of several process parameters (binder
the solution onto the powder bed. This process is repeated saturation, layer thickness, and feed-to-powder ratio) and
layer by layer until a final part is achieved to produce a concluded that achieving higher packing density during
“green part.” On build completion, post-processing steps, powder spreading is critical to achieving higher final density .
[24]
such as curing, depowering, sintering, infiltration, and Lecis et al. also studied the influence of layer thickness, binder
finishing, are necessary to achieve final densification. saturation, as well as debinding and sintering atmospheres to
[12]
Binder jetting offers multiple advantages, such as achieve final densities of 98% .
eliminating the need for support structures, powder The focus of this work lies in the effects of powder
reusability, and part nesting. In contrast to other powder properties, specifically powder size distribution during
AM technologies, it is a non-fusion-based process that binder jetting to achieve superior part properties through
does not rely on high energy sources (e.g., laser and higher density. Prior efforts in powder metallurgy have
electron beam) during fabrication. The only external heat shown that bimodal powder mixtures can improve packing
present during the process is for the partial drying of the density and dimensional control after sintering [14,25] .
binder across each layer. The lack of elevated heat input Both coarse and fine particles are mixed (e.g., 1:3-1:6
is beneficial as it evades melting and rapid solidification volumetric ratios) to increase packing density through
defects, as well as residual stresses accumulated in other filling of fine particles into the voids created between the
AM processed parts . Binder jetting is regarded as a coarse particles . Du et al. investigated the use of bimodal
[1]
[26]
very flexible AM technique that offers a wide range of powder feedstocks in silicon carbide ceramics by achieving
material selections, such as sand, ceramics, polymers, and a 5% increase in green density when compared to unimodal
metals . Applications of binder jetting include tooling, powder prints . Du et al. used spherical alumina powders
[1]
[25]
fuel cells, scaffolds, molds, construction, and electronic to demonstrate the improvements in powder bed density
antenna [2-11] due to its ability in fabricating relatively and sintered density with an analytical model to find the
complex geometries rapidly at larger volumes, and lower optimal mixing fraction in bimodal mixtures . Bai et al.
[27]
machine and production costs . The lack of distortions experimentally evaluated the effect of bimodal copper
[12]
introduced to the part due to the absence of thermal printed parts and observed an increase of 16.2% in powder
gradients, and the lack of thermal crack formations makes bed density and 12.3% in sintered density, depending
this AM technology attractive for continuous investigation on the variation of sinter conditions . Bai et al. studied
[15]
in academia and industrial applications. When compared the impact of copper bimodal mixtures that resulted in
to LPBF and DED, the number of materials investigated in an 8.2% improvement in powder packing density and
binder jetting is smaller but that is increasing . a 4% increase in sintered density . Sinterability and
[13]
[19]
Despite the high popularity of binder jetting, especially for density improvements were also observed in bimodal size
optically reflective and thermally conductive metals , one of distribution in binder jetting of SS 316L but its effects on
[14]
[20]
its limitations is the relatively lower densities of printed parts mechanical strength were not evaluated .
when compared to fabrications through powder metallurgy Even though the impact of bimodal distributions in green
or other metal AM processes . Binder jetting as-built and sintered densities has been explored, there is still a need
[15]
(green stage) parts are typically brittle, porous, and with to understand the effect of bimodal particle size distribution
lower mechanical properties . The previous work reported in SS316L binder jetting and its impact on sintered density
[16]
average relative densities obtained of around 40 – 60% in and mechanical performance. The motivation of this
binder jetting fabrications [16-18] . Efforts have been made to paper is to experimentally validate the benefits of bimodal
increase the overall density of printed part in post-processing, mixtures on sintered density and mechanical performance.
such as infiltration, hot isostatic pressing (HIP), optimization This effort builds on a previous computational work based
of printing process parameters, and powder properties . on discrete element method and the effects of particle size
[15]
For example, Vogt et al. reported a 26% increase in green distributions on packing density, porosity, and flowability .
[28]
density after infiltration while Porter et al. calculated a 65% In addition, the experimental work of this paper serves as
[19]
density improvement when compared to green parts during a benchmark for the modeling work, which provides an
the fabrication of Al-based metal matrix nanocomposites . atomistic level insight into the strengthening mechanism
[20]
Kumar et al. demonstrated that through the use of the HIP of the bimodal particle size distribution using the ReaxFF
technique, a maximum density of 97% and 92% in copper molecular dynamics (MD) simulations.
parts could be achieved [21,22] . Another study showed that
full densification was achieved with HIP in binder jetting of Section 2 describes the experimental methods with
nickel-based superalloy . Optimization process parameters a focus on material selection, powder characterization,
[23]
have also been studied for density improvement. Shrestha et part fabrication and printing conditions, as well as post-

Volume 1 Issue 3 (2022) 2 https://doi.org/10.18063/msam.v1i3.20